Radial flow impellers find application in gas turbine engines where they are used as compressor impellers and turbine impellers. Another application is in the expansion of gases for cooling in refrigeration plants and in gas liquefication plants. Radial flow impellers are greatly subject to structural constrictions in design because of aerodynamic considerations.
In a radial turbine impeller, gas flows into the impeller in a radial direction, entering channels formed by the impeller hub and the impeller blades. Typically, to achieve high aerodynamic performance, the impeller blades at their outer extremities have an integral shroud which forms the outer boundary of the fluid flow channels. The gas is expanded and turned in the impeller from the radial direction to discharge in the axial direction. Thus, the discharge face of the impeller is a generally radial plane, and the blades edges are radial. The blade edges define a large exit area for the expanded axial flow. Consequently, this face is termed the impeller eye. To provide the large exit area, the blade edges have a large radial span. Since these edges in a turbine impeller are trailing edges, they must be thin to provide good aerodynamic performance.
Stresses concentrate at the hub of the blade at the trailing edge. This location is therefore susceptible to cracking, and is critical in establishing the cyclic life of the impeller. Centrifugal stress is a large portion of the total stress at this critical location. The outer shroud is a large contributor to this centrifugal stress. Unshrouded impellers, on the other hand, do not experience such severe stress at this critical location, but have the disadvantage of significantly poorer aerodynamic performance.
The prior art has attempted to reduce stresses at the critical location by configuring the blade geometry. One technique has been simply to use thick trailing edges with attendant poorer aerodynamic performance. To reduce the aerodynamic performance penalty, the thickness of the blade trailing edge has also been tapered, that is, progressively reduced in thickness from the hub of the blade to the tip of the blade. Stress is reduced in that the mass of blade material exerting centrifugal force on the critical location is reduced.
Another technique used in the prior art has been to locate an annular recess on the eye face of the impeller hub at a radius somewhat less than the radius where the blades begin. This annulus introduces some flexibility into the connection of the blade edge with the hub at the eye face, thereby reducing stress in the blade edge at its intersection with the hub. This is especially true for combined blade and shoud material removal where the anticipated aerodynamic efficiency loss for a 5.degree. bevel is only 0.25%.